who is mr. kumar? why is he important in this story?

There are two Mr Kumars. I'm going to assume that the one you're interested in was Pi's teacher.

Satish Kumar (teacher)

Satish is Pi's biology teacher at Petit Seminarie, an active Communist, and a weird-looking man with a bald and pointy head. He is Pi's favorite teacher, and the first avowed atheist that Pi ever meets. A great fan of the zoo, he sees it as his temple.

Satish Kumar (baker)

Satish Kumar is a Muslim mystic and baker, with the same name as Pi's favorite biology teacher. Satish the baker teaches Pi about Islam. He is poor, but very kind and generous. He is so plain-looking that Pi does not always recognize him.